The present invention relates to apparatus and a method for the molding of a load-bearing element made of reinforced thermoplastic, and to one such load-bearing element.
The present invention relates to the technical fields of thermoforming of so-called sandwich elements, and the blow-molding of hollow bodies made of reinforced thermoplastic. The present invention uses thermoplastic layers in a material charge as a plastic membrane which, upon internal pressurization or external vacuum generation, is non-permeable. With the aid of pneumatics and/or hydraulics, the plastic membrane provides a molding force, in the direction towards the internal surfaces of a molding tool, on reinforcement material which lies between the plastic membrane and the internal walls of the molding tool. By heating the thermoplastic matrix material, either before or during molding, to a temperature exceeding the softening temperature, a material charge can be molded and consolidated inside the molding tool in order to provide a load-bearing element made of reinforced thermoplastic.
Load-bearing elements according to the present invention are advantageously used for load-bearing components in vehicles, but they can also be used for other applications where low weight and high rigidity are required.
At present, it is normal for load-bearing components, such as for motor vehicles, to be made from fiber-reinforced thermoplastics. In this connection, use is often made of so-called GMT technology, which involves a material charge with reinforcement material and thermoplastic matrix material being molded by compression molding during heating (thermoforming) in a two-part molding tool.
According to the prior art such a molding tool comprises a number of tool components which can be maneuvered so that the molding tool can be opened and closed. The tool components of the molding tool are additionally arranged so as to be able to apply an external compression force on the material charge which is located inside a cavity inside the molding tool. Moreover, the tool components are provided with guide pins or the like for achieving a good fit between the tool components during compression.
During compression molding, according to the prior art, the thermoplastic matrix material is preheated, with the aid of separate heating means, to the softening temperature or melt temperature of the thermoplastic matrix material.
In the prior art, the material charge sometimes has the form of a so-called preform, which is produced in a separate stage. The preform can be produced, for example, by coating a preforming screen, having a shape which is approximately similar to the final component, with cut roving or other glass fiber material and a suitable binder for holding the reinforcement in place. A precondition of GMT technology is that a thermoplastic matrix material is also present.
The previously known GMT technology requires the use of molding tools which, for example with the aid of hydraulic means, exert a pressure from outside onto the material layers which are to be molded. GMT technology uses a fairly large amount of energy and is less well suited for the manufacture of objects having the lowest possible weight.
Another previously known technique for the molding of components which comprise thermoplastics is blow-molding. In this connection, the raw material is plasticized in an extruder and is formed into a tubular blank which is engaged over a blow nozzle and enclosed in a mold consisting of two concave halves, after which the raw material is blown up and pressed against the cooled walls of the mold cavity and set. This technique has previously been used for the production of light thermoplastic hollow bodies without reinforcement, which hollow bodies therefore have a relatively low degree of wall rigidity.
Thus, a first object of the present invention is to make available an arrangement for molding a load-bearing element of reinforced thermoplastic in an energy-saving manner.
In accordance with the present invention, this and other objects have now been realized by the invention of apparatus for molding a load-bearing element comprising reinforced thermoplastic material provided from a material charge comprising at least one layer of thermoplastic matrix material comprising a non-permeable plastic membrane having a predetermined softening temperature and a reinforcement material, the apparatus comprising a molding tool including a first molding surface and a second molding surface whereby when the first and second molding surfaces are juxtaposed in a closed molding position at least one internal cavity having dimensions corresponding to the load-bearing element is formed therebetween, means for urging the first and second molding surfaces into the closed molding position, heating means for heating the matrix material to a temperature above the predetermined softening temperature, and matrix material molding means for providing a pressure or vacuum force for urging the matrix material against at least one of the first and second molding surfaces with the reinforcement material therebetween to thereby urge the reinforcement material against the at least one of the first and second molding surfaces to mold and consolidate the material charge. In a preferred embodiment, the matrix material molding means provides the force by pneumatic or hydraulic pressure.
In accordance with one embodiment of the apparatus of the present invention, the matrix material molding means provides the force in the form of a pneumatic vacuum.
In accordance with another embodiment of the apparatus of the present invention, the first and second molding surfaces are juxtaposed to provide a molding force onto selected areas of the material charge when in the closed molding position.
In accordance with another embodiment of the apparatus of the present invention, the apparatus includes cooling means disposed within the molding tool. In another embodiment, the apparatus includes heating means disposed within the molding tool.
In accordance with another embodiment of the apparatus of the present invention, at least one of the first and second molding surfaces includes compressing means for compressing predetermined regions of the material charge so as to provide a plurality of internal cavities between the predetermined regions and to provide compressed regions of the material charge including the non-permeable plastic membrane.
In accordance with the present invention, a method has also been devised for molding a load-bearing element comprising providing a molding tool including a first molding surface and a second molding surface forming a closed molding position when the first and second molding surfaces are juxtaposed against each other, introducing a material charge between the first and second molding surfaces comprising at least one layer of thermoplastic matrix material comprising a non-permeable plastic membrane having a predetermined softening temperature and a reinforcement material, with the reinforcement material disposed between the thermoplastic matrix material and at least one of the first and second molding surfaces, molding the material charge between the first and second molding surfaces with areas of the thermoplastic matrix material clamped within the molding tool to provide a seal, heating the material charge to an elevated temperature exceeding the predetermined softening temperature, providing pressure or vacuum force for urging the matrix material against the at least one of the first and second molding surfaces with the reinforcement material therebetween to mold and consolidate the material charge, and cooling the load-bearing element to a temperature below the predetermined softening temperature.
In accordance with one embodiment of the present invention, the at least one layer of thermoplastic matrix material and the reinforcement material are separate layers, whereby during providing of the pressure or vacuum force the at least one layer of thermoplastic matrix material can move with respect to the reinforcement material and thereby press the reinforcement material against the at least one of the first and second molding surfaces to facilitate the molding and consolidation of the material charge.
In accordance with a preferred embodiment, the heating of the material charge to the temperature greater than the predetermined softening temperature comprises heating the material charge to a temperature of between 180xc2x0 C. and 250xc2x0 C.
In accordance with one embodiment of the method of the present invention, cooling of the load-bearing element to the temperature below the predetermined softening temperature comprises cooling the load-bearing element to a temperature of less than 180xc2x0 C.
In accordance with another embodiment of the method of the present invention, providing of the force comprises providing a pneumatic vacuum thereto.
In accordance with another embodiment of the method of the present invention, molding of the material charge includes exerting a molding force from at least one of the first and second molding surfaces against the material charge therewith.
In accordance with another embodiment of the method of the present invention, cooling of the load-bearing element includes cooling through the walls of the molding tool.
In accordance with another embodiment of the method of the present invention, heating of the material charge comprises heating through the walls of the molding tool.
In accordance with another embodiment of the method of the present invention, introducing of the material charge includes at least partially covering the surface of the matrix material with the reinforcement material comprising at least one sheet of a porous fibrous material between the matrix material and the at least one of the first and second molding surfaces.
In accordance with another embodiment of the method of the present invention, the molding tool has a predetermined cross-sectional area, and introducing of the material charge includes providing at least one layer of a film or foil of the matrix material having a surface area greater than the predetermined cross-sectional area of the molding tool, the matrix material comprising polypropylene, polyester, polybutylene terephthalate and polyamide.
In accordance with another embodiment of the method of the present invention, the method includes incorporating at least one additional load-bearing element in the material charge.
In accordance with another embodiment of the method of the present invention, the method includes compressing predetermined regions of the material charge so as to provide a plurality of internal cavities between the predetermined regions and compressed regions of the material charge at the predetermined regions including the non-permeable membrane.
In accordance with the present invention, a load-bearing element has also been provided comprising reinforced plastic including a thermoplastic matrix and reinforcement material simultaneously consolidated during a molding process wherein a plurality of layers of the reinforced plastic surround at least one cavity formed during the molding process by the application of a pressure or vacuum force on the thermoplastic matrix material in a heated state, the thermoplastic matrix material comprising a non-permeable plastic membrane which presses the reinforcement material against the surfaces of the molding tool during the molding process.
In a preferred embodiment, the load-bearing element comprises a first layer of the reinforced plastic and a second layer of the reinforced plastic, the first and second layers of the reinforced plastic being in contact with each other at at least one predetermined contact area and being separated from each other outside the at least one predetermined contact area, thereby forming solid consolidated layers of the reinforced plastic at the at least one predetermined contact area.
In accordance with one embodiment of the load-bearing element of the present invention, the plurality of layers of the reinforced plastic surround a plurality of cavities separated by compressed regions of the reinforced plastic.
In accordance with one embodiment of the load-bearing element of the present invention, the reinforcement material comprises reinforcement fibers, and the majority of the reinforcement fibers are oriented in the primary direction of expected stresses to be applied to the load-bearing element.
In accordance with another embodiment of the load-bearing element of the present invention, the reinforcement material comprises continuous fibers.
In accordance with another embodiment of the load-bearing element of the present invention, the reinforcement material comprises glass fibers.
In accordance with another embodiment of the load-bearing element of the present invention, the matrix material comprises a polymer such as polypropylene, polyester, polybutylene terephthalate and polyamide.
In accordance with another embodiment of the load-bearing element of the present invention, the load-bearing element includes additional load-bearing elements of the reinforced thermoplastic material incorporated therein.
In accordance with another embodiment of the load-bearing element of the present invention, the load-bearing element is incorporated in a component for a motor vehicle.
The objects of the present invention are achieved by means of the fact that the apparatus according to the present invention comprises a two-part molding tool whose internal walls, when the molding tool is in a closed position, form one or more internal cavities with dimensions corresponding to the external dimensions of a load-bearing element which is to be molded; by the fact that the apparatus further comprises means for fixing the molding tool in a closed position, and a material charge which comprises at least one layer of thermoplastic matrix material and reinforcement material; by the fact that the apparatus in this case comprises heating members arranged to heat the matrix material to a temperature which exceeds its softening temperature; by the fact that the apparatus is designed such that areas of the matrix material are clamped securely in the molding tool to provide a seal; and by the fact that the apparatus comprises a means which generates a molding force and which is arranged to act on the matrix material by pressurization and/or vacuum generation, the matrix material serving as a non-permeable, plastic membrane which is arranged to press the reinforcement material against the internal walls of the molding tool and thereby effect molding and consolidation of the material charge.
Another object of the present invention is to make available a method for energy-saving molding of a load-bearing element of reinforced thermoplastic using the apparatus according to the present invention.
This object is achieved by means of the fact that the method comprises opening a two-part molding tool, introducing a material charge with reinforcement material and at least one layer of thermoplastic matrix material, fixing the molding tool in a closed position surrounding the material charge, molding the material charge along the internal surfaces of the molding tool with the material charge at a temperature which exceeds the softening temperature of the matrix material, and opening the molding tool and, after cooling to a temperature below the softening temperature of the matrix material, removing a finished load-bearing element made of reinforced thermoplastic from the molding tool in an open position, by the fact that areas of the matrix material are clamped securely in the molding tool to provide a seal; and by the fact that a means for generating a molding force acts on the matrix material by pressurization and/or vacuum generation, the matrix material serving as a non-permeable, plastic membrane which presses the reinforcement material against the internal surfaces of the closed molding tool and thereby effects molding and consolidation of the material charge.
Another object of the present invention is to make available a load-bearing element of reinforced thermoplastic with high rigidity and low weight.
This object is achieved by means of the fact that the load-bearing element according to the present invention comprises reinforcement material and thermoplastic matrix material, the load-bearing element having been formed by molding and simultaneous consolidation of the reinforcement material and the matrix material; by the fact that layers of reinforced thermoplastic of the load-bearing element surround at least one cavity which has been formed during molding by applying an internal pressure and/or an external vacuum on the matrix material in a heated state, the matrix material having served during molding as a plastic membrane which is non-permeable upon pressurization or vacuum generation; and by the fact that the membrane during molding has pressed the outerlying reinforcement material against the internal surfaces of a surrounding molding tool.
According to the present invention, the matrix material and the reinforcement material are made available in the form of separate layers which, during molding, can move relatively freely in relation to one another, while the molding force is essentially generated with the aid of the means for generating a molding force. All in all, this means that relatively little energy is consumed in the molding operation.
Rigidity and load-bearing capacity can be optimized by means of the fact that the arrangement and method according to the present invention make it possible, for example, to arrange separate pieces of reinforcement material of the material charge in such a way that the reinforcement fibers run essentially in the direction in which the expected loads will have their direction of stressing in the load-bearing element.
Moreover, the weight of the load-bearing element can be optimized, while taking into consideration the rigidity and load-bearing capacity, by virtue of the cavities which, according to the present invention, can be created at suitable sites within the material structure.